During medical procedures, locating and accessing a body cavity is often a required step for therapeutic, anesthetic, and diagnostic purposes. For example, procedures such as epidural injection, emergency tracheotomy, chest tube drainage, and percutaneous gastrostomy fistula all require accurately locating and accessing an implicated body cavity with a needle. These puncturing procedures bear high risk factors. When the needle is inserted too far or not far enough, catastrophic consequences can often result.
Using epidural injection as an illustrative example, this procedure requires positioning the tip of a needle precisely in the epidural space (also known as the epidural cavity) in order to administer pain-killing medicine. The epidural space is an anatomic space located at the outermost portion of the spinal canal, lying in between the ligamenta flava and the dura mater. In humans, the epidural cavity contains lymphatics, spinal nerve roots, loose connective tissue, fatty tissue, small arteries, and a network of internal vertebral venous plexuses. It is only about 3-5 mm wide. Inserting a needle beyond the epidural space can easily cause spinal cord injuries and other medical complications. Hence, it is very important to be able to accurately locate this body cavity.
Conventional methods for locating the epidural space generally rely on the manual skills of the operator (e.g. anesthesiologist, nurse, etc.). To perform an epidural injection using a conventional method, the operator will have to place the patient in a proper position first. Next, the operator will insert a needle into the spinal column of the patient. Once the tip of the needle enters the deeper part of the interspinous ligament, the operator will hold the needle in place and attach a saline or air filled syringe to the bub of the needle. Along with the syringe, a shaft or wing-shaped handle is typically mounted on the hub for the operator to apply an advancing force to drive the needle further into the body. As the operator pushes on the handle to move the needle forward, he will also push on the plunger of the syringe at the same time to maintain a pressure within the syringe barrel. Before the needle reaches the epidural space, there is no place for the air or saline to go, so a back pressure will be built up within the syringe barrel. This way, the operator will feel a sudden loss of resistance (LOR) when the needle reaches the epidural space as the air or saline is ejected into the epidural space. This sudden LOR allows the operator to know when to stop advancing the needle.
The above described conventional method is highly subjective, technically demanding, and prone to human error. It also takes much longer to locate the space as the operator must do it slowly to avoid missing the 3-5 mm space. Any slight inertial movement of the hand can easily move the needle pass beyond the epidural space, causing dural puncture and nerve damage.
Numerous attempts have been made in the art to address this problem. One approach focuses on providing a visual cue to assist the operator in knowing when the needle has reached the epidural space. For example, U.S. Pat. No. 7,175,608 B2 to Hasan teaches a representative device for locating the epidural space that utilizes a diaphragm to provide a visual cue to the operator for indicating when the needle has reached the epidural space. In Hasan's device, the diaphragm is adapted for pressurization such that when the device is pressurized, it bulges outwards to provide a visual indicator of the pressure change. Many other prior art methods and devices teach various means for similarly providing visual cues to indicate that the needle has reached the epidural space. However, these devices and methods do not solve the fundamental problem of operator error because they still must rely on the human operator to heed the warning and stop advancing the needle.
U.S. Pat. No. 8,715,234 B2 to Bethi teaches a device for locating a needle in a body cavity that purports to be able to automatically stop advancing the needle once the needle has reached the body cavity. The device works by balancing the frictional force between the needle and the ligament against the back pressure within the syringe barrel. It includes a syringe encased by a frame that is solely connected to the plunger of the syringe. While the needle is in the ligament, the back pressure within the barrel of the syringe will exceed the frictional force between the needle and the ligament so that when the operator pushes the plunger forward, the back pressure will act to advance the entire syringe/needle assembly forward. Upon entering the body cavity, the content of the syringe barrel will be released, thereby relieving the back pressure. At this point, the needle will be held in place by friction between the needle and the ligament while the plunger will advance to empty the contents of the syringe.
While the device of Bethi purports to be able to stop the needle upon entering the targeted cavity, it requires bulky construction and is not compatible with existing setup commonly found in medical facilities. In actual practice, Bethi's device further suffers from several other drawbacks. For example, because Bethi's device relies on balancing the air pressure in the syringe barrel against the friction between the needle and the ligament, the compressible nature of air will create an inherent instability in the needle-syringe assembly. This means that fine control of the needle is basically impossible, making it difficult for a user to precisely place the needle and further increasing the risk of damaging surrounding tissues. Moreover, the device relies on the negative pressure difference between the target cavity and the ligament; hence, it only works for cavities that have negative pressures relative to the ligament (e.g. the epidural space). For cavities that have higher pressure relative to the ligament (e.g. chest cavity), Bethi's device will not work.
Therefore, there still exists an urgent need for more universally applicable methods and devices of locating body cavities and placing needles therein without high risk of puncturing surrounding vital structures.